Apparatus for improving the signal information in the electron beam examination of sample topography

ABSTRACT

In order to enhance the current of secondary electrons flowing from a sample scanned by a primary electron beam, there are provided means to generate at the sample an electric field of such intensity and direction that a substantial portion of the secondary electrons emitted by the primary electron bombardment is drawn away from the sample surface. The electric field may be periodically varied. The sample current is applied to an image forming apparatus correlated to the scanning motion of the primary electron beam.

United States Patent 1 Weber et al.

1 1 May 29, 1973 [54] APPARATUS FOR IMPROVING THE SIGNAL INFORMATION IN THE ELECTRON BEAM EXAMINATION OF SAMPLE TOPOGRAPHY [75] Inventors: Ulrich Weber, 7500 Karlsruhe 21;

Jiirgen Gullasch, 6741 Minderslachen, both of Germany [73 Assignee: Siemens Aktiengellschaft, Munich,

Germany 221 Filed:' Feb.8,l97l 21 Appl.No.: 112,919

[30] Foreign Application Priority Data Feb. 7, 1970 Germany ..P 20 05 682.0

[52] U.S. Cl. ..250/49.5 PE, 250/495 A [51] Int. Cl. ..G01n 23/22 [58] Field of Search ..250/4l.9 D, 49.5 R,

' 250/49.5 A, 49.5 PE

[56] References Cited UNITED STATES PATENTS 3,103,584 9/1963 Shapiro et al. ..25o/49.5 3,474,245 10/1969 Kimu'ra et al ..25o/49.5 3,315,157 4/1967 Watanabe et a1 ..250/49.5 X 3,158,733 11/1964 Sibley ..2so/49.5 x 3,291,959 12/1966 Schleich et al. ..25o/49.5 x

Primary Examiner-William F. Lindquist Attorney-Edwin E. Greigg [57] 1 ABSTRACT In order to enhance the current of secondary electrons flowing from a sample scanned by a primary electron beam, there are provided means to generate at the sample an electric field of such intensity and vdirection that a substantial portion of the secondary electrons emitted by the primary electron bombardment is drawn away from the sample surface. The electric field may be periodically varied. The sample current is applied to an image forming apparatus correlated to the scanning motion of the primary electron beam.

13 Claims, 5' Drawing Figures i 5m 1 MAT/1V6 Patented May 29, 1973 3 Sheets-Sheet. 1.

EVAL 1/47746 ME/l/VS Fig.2

EVALUAT/A G MFA/V5 Patented May 29, 1973 3 Sheets-Sheet I5 EVAL 4/4 T/A G 'IIIIIJ H' MEANS Fig. 5

BACKGROUND OF THE INVENTION mary electron beam.

With the aid of the scanning electron microscopy the external configuration and topography of a sample surface may be determined, while by means of the electron probe X-ray microanalysis an elementary analysis of the inner structure of objects having a dimension of only a few micrometers may be preformed. In the scanning electron microscopy mostly low-energy secondary electrons (approximately 50 eV maximum value) are used as measuring (sensed) signals; they are emitted by the sample surface upon bombardment thereof with high energy (SO-40 KeV) primary electrons. On the other hand, in the electron probe X-ray microanalysis practically all physical occurrences are used as possible sources of information for the analysis.

A magnetic deflecting system incorporated in a electro-optical system of the electron beam generator permits, in addition to the dot analysis by means of a linewise deflection of the primary electron beam over the sample surface and evaluating devices guided synchronously therewith (such as recording, plotting and image forming apparatus), the measuring and plotting of the course of intensity of the measuring signals along lines or over a range of the sample surface.

Upon impingement of the electrons of the primary electron beam on the sample surface, mostly X-ray beams, back-scattered electrons and secondary electrons are emitted.

In the X-ray analysis a dot resolution in the order of magnitude of 1 micron is obtained for samples of large mass. In case of thin preparations which are transparent to primary electrons, a substantially smaller dot resolution may be obtained. On the other hand, the backscattered electrons which retain a substantial part of the primary energy, yield in the most favorable case with a sample of large atomic number, a dot resolution down to 0.2 micron. It is only the secondary electrons which, because of their very small energy, have a very small range in the sample and therefore have a diffusion range which affects the resolution only when the primary electron beam has a diameter which is below a few times millimicrons. For this reason the secondary electrons are used in the scanning electron microscopy for the image formation of the surface configuration of the sample in case the primary electron beam has adiameter as small as 10 millimicrons and in such 7 a case they make possible a dot resolution of -20 millimicrons. Thus, the dot resolution during the analysis of a sample surface with the .aid of secondary electrons instead of'back-scattered electrons is better by one order of magnitude.

It is known in the electron probe X-ray microanalysis to sense also the so-called sample current I, of a sample scanned in a screen-like manner by the primary electron beam, amplify it in a sample current amplifier and then utilize it for the image formation. The sample current I, is obtained from the current 1, of the primary electrons by subtracting the current I, of the back scattered electrons as well as the current I of the secondary electrons generated at the surface of the sample. Thus:

n o r' |e (1) Generally, the component of the secondary electrons in the sample current I, is neglible since'the secondary electrons generate a space charge in the vicinity of the impingement of the primary electrons, so that only a small fraction of the generated secondary electrons may leave the sample surface.'Thus, the sample current I, is substantially the difference between the current I, of the primary electrons and the current I of the backscattered electrons. The current I, of the primary electrons is maintained constant by the electron beam generator. The resolution of the sample imageobtainable by the sample current is thus determined in the first place by the back-scatteredelectrons. In case of radiation-impervious, large-mass samples, even if the diameter of the primary electron beam is reduced, the resolution of the sample current image will be several times 0.1 micron. This value corresponds to the diameter of the diffusion range of the (rapid) primary or back scattered electrons. A detailed contrast and a good resolution which in the scanning-type electron microscopy is achieved by utilizing the secondary electrons for the formation of the pictures could heretofore not be achieved in sample current images for the physical reasons set forth hereinabove.

It is further known that the emission of the secondary electrons may be substantially varied by applying an electric voltage to the sample.

In the scanning electron microscopy it has been customary to at least partially separate the secondary electrons from the other electrons by means of a secondary electron detector and indicate their magnitude. The secondary electron detector is, however, a very expensive apparatus: itcomprises an electro-optical system, a scintillation crystal, a photomultiplier, a high voltage supply device and an after-connected amplifier.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to provide an apparatus for the scanning electron microscopy and the electron probe microanalysis which is of a simpler structure, is easier to operate and is more economical than a secondary electron detector and, contrary thereto, utilizes for the image formation indirectly all secondary electrons emitted by the sample.

Briefly stated, according to the invention, to effect the formation of a contrast-rich, high-resolution image of the surface structure of a sample, there areprovided means for generating at the sample surface an electric field of such a direction and intensity that all, or a substantial portion, of the secondary electrons which are emitted by the sample upon its bombardment with the primary electron beam and which leave the sample surface may be led away. There is further provided an evaluating apparatus which is correlated to the scanning motion and to which there is applied a sample current taken from the sample.

The invention will be better understood and further objects as well as advantages of the invention will become more apparent from the ensuing detailed specification of several embodiments taken in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWING r the invention;

FIG. 4 is a circuit diagram of a fourth embodiment of the invention and FIG. 5 is a circuit diagram of a fifth embodiment of the invention.

GENERAL DESCRIPTION OF THE BASIC INVENTION AND ITS MODIFICATIONS According to the invention there is provided an apparatus which effects a certain electric potential distribution about thesample and thereby substantially or completely prevents the buildup of space charges in front of the sample surface. All, or at least a substantial part, of the secondary electrons emitted by bombardment with the primary electron beam and also with the backscattered electrons are drawn away by the electric field and may thus leave the sample surface. It is noted that, dependent upon the properties of the sample, the strength of the electric field is expediently between 100 and 1,000 V/cm. In equation (1), therefore, the current I of the secondary electrons may no longer be neglected. The magnitude of the secondary current I is now comparable to the current I, of the primary electrons and, under certain circumstances, may even be substantially larger. The current intensity I, of the back-scattered electrons, on the other hand, has a value of only a fraction of the current intensity I, of the primary electrons (1,, 0.1-0.4). In case of a uniformly structured sample, I, remains substantially constant during the entire scanning process. Thus, according to equation (1), the sensed sample current I, contains, in addition to a D.C. component determined by the number of the primary and the back-scattered electrons, substantially the signal which is carried by the current I of the secondary electrons and which constitutes the information regarding the surface structure of the sample.

If in a scanning electron microscope or in an electron probe microanalyzer, the sample current I, taken from the sample by the aforeoutlined means is applied to an evaluating device for the purposeof image formation, the surface structures of the sample may be represented with similar or identical resolution and identical contrast as in conventional scanning electron microscopes which use secondary electron detectors according to the prior art. In sample current amplifiers, on the other hand, conventionally used in electron beam analyzers, without means for drawing the secondary electrons away from the sample face, only a substantially inferior resolution and a substantially smaller image contrast may be achieved.

In order to support the removal of the emitted secondary electrons, an additional apparatus may be provided according to the invention for generating a magnetic field on the sample face. It is expedient to choose the magnetic field weak (approximately 10-100 oersteds) and constant in time and to apply it parallel to the sample face.

On the sample face there is provided an electric contact for the takeup of the sample current I,,. The latter or a voltage proportionate thereto, may be applied to the evaluating device through an amplifier.

It is seen that in equation (1) the secondary electron current I has a negative sign. Therefore, the surface image produced by the evaluating device may be a negative representation. In order to avoid such an occurrence and thus ensure the formation of a positive image, according to the invention the voltage proportionate to the sample current I, is applied to the input of the amplifier or directly to the input of the evaluating device after reversing its polarity.

For generating the electric field on the sample face according to the invention, there may be applied a potential to one or more components of the electron microscope or electron probe microanalyzer immediately in front of the sample face. This potential is positive with respect to the sample and affects only slightly, if at all, the primary electron beam. The said component may be a lens pole shoe of the fine beam lens disposed adjacent the sample or may be a wall of the sample chamber. The same purpose may be achieved by grounding the components (for example, the lens pole shoe or the sample chamber wall) and connecting the electric contact provided on the sample face to an input terminal of the evaluating device or the amplifier. The last-named input terminal is at a potential which is negative with respect to the said components.

The electric field on the sample face may be generated in still another manner. Thus, according to the invention, there is provided an electric field generating device comprising two electrodes; the first electrode is connected with the negative pole and the second electrode is connected with the positive pole of a variable voltage source in such a manner that every electron on the sample face is exposed to a force component directed outwardly away from the sample face.

The first electrode may be a metal plate, a metal sieve, a sample support, or may be constituted by the sample itself. As a further alternative, the sample may be partially coated with an electrically conductive layer which constitutes the first electrode. In the latter case, the electric contact provided on the sample or on the electrically conductive layer for drawing the sample current I should be connected through a work resistance with the negative pole of the voltage source.

The second electrode may have a knife edge configuration disposed in a plane that is normal to the direction of the primary electron beam. In the alternative, the second electrode may be an electrically conductive plate provided with a bore hole for allowing the passage of the primary electron beam. The diameter of said bore hole is so dimensioned that the primary electron beam passing therethrough is not appreciably affected by the positive potential applied. The second electrode may thus be formed as an annular disc which is arranged concentrically with respect to the direction of the primary electron beam. For certain purposes it is expedient to give the second electrode a pointed configuration.

The image contrast is a function of the angle of incidence at which the electric field lines enter the sample face. Thus, if said angle of incidence is varied, the image contrast changes. For the purpose of rendering this angle of incidence variable, the second electrode is displaceable in a plane normal to the direction of the primary electron beam or in a plane parallel to the surface of the sample.

which, in turn, is positive'with respect to the sample face. In this case too, it is expedient to arrange the third electrode in such a manner that it is displaceable in a plane normal to the direction of the primary electron beam or rotatable about an axis parallel to the primary electron beam. The third electrode may be further disposed in such a manner that in case of tilting, shifting or rotating of the sample, its position relative to the sample face remains unchanged.

For numerous problems of analysis it is sufficient if the electric field and the additional magnetic field applied to the sample face is constant in time. If one of these fields is periodically varied, then a further increase in the resolution and in the image contrast may be achieved. This is so because in the evaluating apparatus exclusively the signal of the secondary electrons is used for the image formation, whereas signal components originating from back-scattered electrons or primary electrons are completely withheld. Accordingly, there are provided means generating on the sample face an additional magnetic field controllable by a function generator for periodically weakening or interrupting the secondary electron current I, generated at the sample face. The function generator may be a square wave or a sine wave generator.

The above phenomenon may be explained by the fact that in the sample current according to equation (1) only that portion has electrons of very low kinetic energy which is formed by the secondary electrons leaving the sample face. These electrons, for example by means of applying a periodical electric potential to the sample, are alternately allowed to leave and prevented from leaving the sample face. The modulation affects exclusively the secondary electrons and has no effect on the high-energy primary electrons and backscattered electrons.

Thus, the aforenoted arrangement according to the invention comprises a chopper by means of which the outflow of the secondary electrons from the sample is periodically prevented. Such a chopper effect may be achieved, for example, by applying a sufficiently high A.C. voltage between the sample and an'adjacent portion of the lens pole shoe or the sample chamber wall. Further, the amplifier is to amplify only those sample current components which vary periodically with the chopper frequency. According to the invention, before the amplifier there is connected a narrow band pass filter tuned to the frequency of the function generator. It is even more expedient to ensure that only those sample current components are admitted to the evaluating device which vary in phase with the chopper frequency. For this purpose, according to the invention, the amplifier is replaced by a phase responsive rectifier with a preamplifier which may be coupled capacitively or inductively at its input side to the sample contact for theseparation of a D.C. component. The phase responsive rectifier is expediently tuned to the frequency of the function generator. It comprises, for the adjusting of amplitude, a phase adjusting component.

Further, according to the invention, both output terminals of the function generator are connected to one another through two serially connected resistances and the second output is connected to the second electrode and the first output is connected to the first electrode, while a tap between the two resistances is grounded.

The secondary electron current I may be modulated by connecting the function generator to a pair of solenoids generating a weak alternating magnetic field parallel to the sample face.

In the apparatus according to the invention a tilting of the sample for the purpose of changing or intensifying the contrast is not always necessary. Such a purpose may be achieved by other means, for example by the already described displacement of the electrodes. A tilting of the sample is, however, useful if the sample is simultaneously to be examined by means of another system, such as an X-ray beam detector, or if certain sample ranges are to be made visible. Therefore, according to the invention, the sample is tiltable through determined angles about an axis normalto the direction of the primary electron beam in the direction of a further analyzing system. Furthermore, it is expedient if the sample is "rotatable about an axis normal to the sample surface.

Further, according to the invention, the sample may be coated by vapor deposition or other means with a thin layer of a material which emits a large number of secondary electrons with respect to the number of the impinging primary electrons. Such a layer thus serves to enhance the emission of the secondary electrons.

DESCRIPTION OF THE EMBODIMENTS WITH REFERENCE TO THE FIGURES Turning now to FIG. 1, a primary electron beam E having a current intensity I, is, after passing through a lens pole shoe L, deflected by known means to scan a sample P. The latter and the lens pole shoe L are surrounded by wall W of a sample chamber. The secondary electrons emitted by the sample P upon bombardment with the primary electron beam E are drawn away from the sample by means of an electric field. For this purpose, the negative pole of a voltage source Q1 is connected through a work resistance R to an electric contact K provided onthe sample P. The latter forms a first electrode E1 of a condenser. The second electrode E2, which is formed of a disc having a bore hole B for allowing the passage of the primary electron beam E, is connected to the positive pole of the voltage source Q1. The wall W of the sample chamber and the negative pole of the voltage source Q1 are grounded.

The electrons flowing from the first electrode El through the contact K and through the work resistance R through the ground generate at the work resistance R a voltage U which is proportionate to the sample current I, and which is applied to an amplifier V. The voltage S appearing at the output of the amplifier V is applied to the input of an evaluating means A which is controlled synchronously with the scanning motion of the primary electron beam E. The evaluating device A may be a registering device, a data supplying device, or an image forming apparatus.

Turning now to FIG. 2, in the embodiment shown therein the positive pole of the voltage source 01 is grounded and is connected to an electrically conductive apparatus component in the immediate vicinity of the sample P, such as the wall W of the sample chamber and the lens pole shoe L. The negative pole of the voltage source Ql is connected with the sample P through the workresistance R. The amplification of the voltage U is effected as in the apparatus according to FIG. 1; care has to be taken, however, that none of the amplifier input terminals is grounded. The sample P is tiltable, by a schematically illustrated means H, through a determined angle a about an axis normal to the direction of the primary electron beam E in the direction of a further analyzing means which may be disposed in the space in front of the sample P.

Turning now to the embodiment depicted in FIG. 3, in the space in front of the sample P, as viewed in the direction of travel of the primary electrons forming the electron beam E, there is disposed an additional electrode E3 for varying the image contrast. The electrode E2-is formed as a knife edge and is movable by means of a' bar D in a plane normal to the primary electron beam E, as indicated by the double-headed arrow G. The third electrode E3, which also has the configuration of a knife edge, may be stationary or may be movable with the electrode E2 as a unit. From a further voltage source Q2 a voltage is applied to the third electrode E3 which is negative with respect to the potential on the second electrode E2. For this purpose the positive pole of the voltage source Q2 is connected with the negative poleof the voltage source Q1 and the negative pole of voltage source Q2 is connected with the third electrode E3. The circuit diagram of the voltage source Q1 and the evaluating system corresponds to that illustrated in FIG. 1

Turning now to FIG. 4, in the embodiment illustrated therein the electric field associated with the sample face is periodically varied by means of a function generator F. For this purpose both output terminals A1 and A2 of the function generator Fare connected to one another through a voltage divider which is formed by two serially connected resistances R1 and R2. The second output terminal A2 is connected to the second electrode E2 (having the same role as in FIG. 1) and the first output terminal Al is connected to an electrode E1. The sample P provided with a contact K is disposed between the two electrodes El and E2. The tap between the two resistances R1 and R2 is grounded and is also connected through the work resistance R with the contact K. The wall W of the sample chamber and the pole piece L are all grounded.

The secondary electrons emitted by the sample P are periodically prevented from leaving the sample and are periodically drawn away therefrom by means of an alternating electric field generated between electrodes E1 and E2. The sample current I, which flows through the work resistance R contains an A.C. component which has the frequency supplied by the function generator F and on which there may be superimposed a D.C. component. The A.C. component alone contains the information delivered by the secondary electrons and concerning the topography of the sample P.

As set forth earlier under the GENERAL DE- SCRIPTION, in addition to an electric field a magnetic field may also be provided at the sample surface for aiding the electric field. It has also been stated that the periodic modulation of the sample current may be effected by varying one of these two fields.

' Accordingly, to implement the aforenoted possibility of applying additionally a magnetic field, there is provided a coil means M which, when energized, generates a magnetic field parallel with the sample surface. The

coil means M has terminals A3 and A4.

Thus, according to one possibility, when the electric field is periodically varied by the function generator F, the terminals A3, A4 may be connected to a D.C. source.

If, on the other hand, it is desired to periodically vary the magnetic field, then the terminals A3, A4 are connected to the function generator F. At the same time, a constant electric field is generated, for example, by connecting the electrode E2 to a D.C. voltage source as illustrated in FIG. 1.

The voltage U taken from across the work resistance R is applied to a phase-responsive rectifier PG with a preamplifier, while the D.C. component is separated by means of a condenser C. The values of the RC member are determined dependent upon the velocity with which the primary electron beam scans the sample P. The phase-responsive rectifier PG with preamplifier is connected with the function generator F for obtaining a reference signal. The signal voltage S1 appearing at the output terminals of the phase-responsive rectifier PG, is applied to the evaluating device A.

Turning now to FIG. 5, the first electrode E1 is formed as a sieve, while the second electrode E2 has a pointed configuration. The second electrode E2 may be displaced normal to the primary electron beam E in the directions indicated by arrow G by means of a rod D. The sample P is disposed between the two electrodes E1 and E2.

To the first electrode El there is applied the grounded negative pole of the voltage source Q1 through a work resistance R, while the second electrode E2 is connected with the positive pole of voltage source Q1. The voltage U across the work resistance R is applied to the evaluating apparatus A with a reversal of polarities. For this purpose, there is provided a onestage amplifier which includes, in a known manner, an electron tube T. It is to be understood that a one-stage transistor amplifier may also be used.

The grid of the electron tube T is connected to the electrode El, while its cathode is grounded. The anode of the electron tube T is connected through an anode resistance R1 with the positive pole of a further voltage source 03, the negative pole of which is grounded. The voltage between the anode and cathode of electron tube T is applied to the evaluating apparatus A. It is 'apparent from the known operation of electron tube T that the voltage applied to the evaluating apparatus A will be more negative if the voltage taken from resistance R increases, since the inner resistance of electron tube T decreases. Converse considerations apply when the voltage U decreases. Thus, the polarity of the voltage applied to the evaluating apparatus A varies in a reverse manner with respect to the polarity of voltage U.

ADDITIONAL ADVANTAGES OF THE INVENTION The apparatus, according to the invention, has a number of advantages listed hereinbelow.

Tertiary electrons have no effect on the image formation (tertiary electrons are those secondary electrons which are emitted by the surface of other solid bodies such as the lower portion of the lens pole shoe or the sample chamber wall in the vicinity of the sample upon bombardment with back-scattered electrons coming from the sample).

All electrons which leave the sample are usedfor evaluation.

Because of its small volume, the'apparatus may be disposed at an advantageous distance with respect to the sample.

The sample may be tilted without loss of resolution, for example, in the direction of an available dispersive X-ray semiconductor spectrometer, so that simultaneously optimal images of different information may be obtained.

The image intensity at the sample surface is substantially independent from stray magnetic fields since the secondary electrons do not pass through any detector opening, but merely have to leave the surface of the sample.

By altering the electric potential distribution in the vicinity of the sample face, particularly by altering the direction of the electric field which draws away the secondary electrons from the sample, the light distribution of nonplanar surface structures of the sample may be varied (variation of the direction of light and shadow).

What is claimed is:

1. In an apparatus for analyzing the topography of a sample by utilizing secondary electron emission by said sample, said apparatus being of the known type that has (a) means for generating a primary electron beam, (b) means directing said primary electron beam onto a surface of said sample to bombard the same with primary electrons causing the emission of secondary electrons in a number of characterizing the intensity of said primary electron beam and the topography of the location bombarded and .(c) means for deflecting said primary electron beam to scan said sample surface, the improvement comprising,

A. electric field generating means for generating at said sample surface an electric field oriented for drawing and leading away from said sample surface at least a substantial portion of said secondary electrons,

B. electric circuit means connected to said sample, said electric circuit means constituting conductor means for said sample current generated by said primary electron beam, the momentary intensity of said sample current being a function of the momentary magnitude of said secondary electron emission by said sample,

C. function generator means controlling said electric field generating means for periodically decreasing the magnitude of said secondary electron emission to give said sample current an A.C. component being alternatingly affected and unaffected by said secondary electron emission with a periodicity determined by the frequency of said function generator means,

D. separating means receiving said A.C. component,

said separating means generating a signal containing information only of those portions of said A.C. component that are affected by said secondary electron emission, said signal characterizing the sample topography scanned,

E. evaluating means receiving said signal from said separating means and F. means for correlating said evaluating means to the scanning motion of said primary electron beam.

2. An improvement as defined in claim 1, including means for reversing the polarity of said signal prior 'to applying it to said evaluating means.

3. An improvement as defined in claim 1, wherein said apparatus includes electrically conductive apparatus components in the immediate vicinity of said sample, said electric field generating means including means for applying a potential to at least one of said electrically conductive apparatus components; said potential is positive with respect to said sample and is without appreciable effect on said primary electron beam.

4. An improvement as defined in claim 1, wherein said electric field generating means includes A. a voltage source,

B. a first electrode connected to one pole of said voltage source and C. a second electrode connected to the other pole of said voltage source; the electric field generated between said electrodes has a force component urging any electron on the sample surface away from said sample.

5. An improvement as defined in claim 4, wherein said second electrode has the configuration of a knife edge and is movable in a plane normal to the direction of said primary'electron beam.

6. An improvement as defined in claim 4, wherein said second electrode is formed of an electrically con ductive plate disposed in a plane transverse to the direction of said primary electron beam, said plate is provided with an opening of a dimension as to allow passage of said primary electron beam without being affected by the potential applied to said plate.

7. An improvement as defined in claim 4, wherein said second electrode has a pointed configuration.

8. An improvement as defined in claim 4, including means to displace said second electrode.

9. An improvement as defined in claim 4, including A. an additional voltage source and B. a third electrode disposed in the space in front of said sample surface when viewed in the direction of said primary electron beam and connected to said additional voltage source; the potential on said third electrode being variable independently from the potential of said second electrode.

10. An improvement as defined in claim 9, including means to move said third electrode with respect to the location of impingement of said primary electron beam on said sample.

11. An improvement as defined in claim 1, said separating means including a phase-responsive rectifier with preamplifier receiving said A.C. component.

12. An improvement as defined in claim 1, including means to move said sample with respect to the direc tion of said primary electron beam.

13. In an apparatus for analyzing the topography of a sample by utilizing secondary electron emission by said sample, said apparatus being of the known type that has (a) means for generating a primary electron beam, (b) means directing said primary electron beam onto asurface of said sample to bombard the same with primary electrons causing the emission of secondary electrons in a number characterizing the intensity of said primary electron beam and the topography of the location bombarded and (c) means for deflecting said primary electron beam to scan said sample surface, the improvement comprising,

A. electric field generating means for generating at said sample surface an electric field oriented for drawing and leading away from said sample surface at least a substantial portion of said secondary electrons,

1 1 12 B. magnetic field generating means for generating at being alternatingly affected and unaffected by said said sample surface a magnetic field oriented for secondary electron emission with a periodicity deaiding said electric field in drawing and leading t r ined by the frequency of said function gener away from said sample surface at least a substantial tor means, portion of said secondary electrons, 5

E. separating means receiving said A.C. component,

said separating means generating a signal containing information only of those portions of said A.C. component that are affected by said secondary electron emission, said signal characterizing the C. electric circuit means connected to said sample, said electric circuit means constituting conductor means for said sample current generated by said primary electron beam, the momentary intensity of said sample current being a function of the momentary magnitude of said secondary electron emission sample by said sample F. evaluating means receiving said signal from said D. function generator means controlling one of said separating means P field generating means for periodically decreasing G. means for correlating said evaluating means to the the magnitude of said secondary electron emission 5 scanning motion of said primary electron beam. to give said sample current an A.C. component 

1. In an apparatus for analyzing the topography of a sample by utilizing secondary electron emission by said sample, said apparatus being of the known type that has (a) means for generating a primary electron beam, (b) means directing said primary electron beam onto a surface of said sample to bombard the same with primary electrons causing the emission of secondary electrons in a number of characterizing the intensity of said primary electron beam and the topography of the location bombarded and (c) means for deflecting said primary electron beam to scan said sample surface, the improvement comprising, A. electric field generating means for generating at said sample surface an electric field oriented for drawing and leading away from said sample surface at least a substantial portion of said secondary electrons, B. electric circuit means connected to said sample, said electric circuit means constituting conductor means for said sample current generated by said primary electron beam, the momentary intensity of said sample current being a function of the momentary magnitude of said secondary electron emission by said sample, C. function generator means controlling said electric field generating means for periodically decreasing the magnitude of said secondary electron emission to give said sample current an A.C. component being alternatingly affected and unaffected by said secondary electron emission with a periodicity determined by the frequency of said function generator means, D. separating means receiving said A.C. component, said separating means generating a signal containing information only of those portions of said A.C. component that are affected by said secondary electron emission, said signal characterizing the sample topography scanned, E. evaluating means receiving said signal from said separating means and F. means for correlating said evaluating means to the scanning motion of said primary electron beam.
 2. An improvement as defined in claim 1, including means for reversing the polarity of said signal prior to applying it to said evaluating means.
 3. An improvement as defined in claim 1, wherein said apparatus includes electrically conductive apparatus components in the immediate vicinity of said sample, said electric field generating means including means for applying a potential to at least one of said electrically conductive apparatus components; said potential is positive with respect to said sample and is without appreciable effect on said primary electron beam.
 4. An improvement as defined in claim 1, wherein said electric field generating means includes A. a voltage source, B. a first electrode connected to one pole of said voltage source and C. a second electrode connected to the other pole of said voltage source; the electric field generated between said electrodes has a force component urging any electron on the sample surface away from said sample.
 5. An improvement as defined in claim 4, wherein said second electrode has the configuration of a knife edge and is movable in a plane normal to the direction of said primary electron beam.
 6. An improvement as defined in claim 4, wherein said second electrode is formed of an electrically conductive plate disposed in a plane transverse to the direction of said primary electron beam, said plate is provided with an opening of a dimension as to allow passage of said primary electron beam without being affected by the potential applied to said plate.
 7. An improvement as defined in claim 4, wherein said second electrode has a pointed configuration.
 8. An improvement as defined in claim 4, including means to displace said second electrode.
 9. An improvement as defined in claim 4, including A. an additional voltage source and B. a third electrode disposed in the space in front of said sample surface when viewed in the direction of said primary electron beam and connected to said additional voltage source; the potential on said third electrode being variable independently from the potential of said second electrode.
 10. An improvement as defined in claim 9, including means to move said third electrode with respect to the location of impingement of said primary electron beam on said sample.
 11. An improvement as defined in claim 1, said separating means including a phase-responsive rectifier with preamplifier receiving said A.C. component.
 12. An improvement as defined in claim 1, including means to move said sample with respect to the direction of said primary electron beam.
 13. In an apparatus for analyzing the topography of a sample by utilizing secondary electron emission by said sample, said apparatus being of the known type that has (a) means for generating a primary electron beam, (b) means directing said primary electron beam onto a surface of said sample to bombard the same with primary electrons causing the emission of secondary electrons in a number characterizing the intensity of said primary electron beam and the topography of the location bombarded and (c) means for deflecting said primary electron beam to scan said sample surface, the improvement comprising, A. electric field generating means for generating at said sample surface an electric field oriented for drawing and leading away from said sample surface at least a substantial portion of said secondary electrons, B. magnetic field generating means for generating at said sample surface a magnetic field oriented for aiding said electric field in drawing and leading away from said sample surface at least a substantial portion of said secondary electrons, C. electric circuit means connected to said sample, said electric circuit means constituting conductor means for said sample current generated by said primary electron beam, the momentary intensity of said sample current being a function of the momentary magnitude of said secondary electron emission by said sample, D. function generator means controlling one of said field generating means for periodically decreasing the magnitude of said secondary electron emission to give said samplE current an A.C. component being alternatingly affected and unaffected by said secondary electron emission with a periodicity determined by the frequency of said function generator means, E. separating means receiving said A.C. component, said separating means generating a signal containing information only of those portions of said A.C. component that are affected by said secondary electron emission, said signal characterizing the sample topography scanned, F. evaluating means receiving said signal from said separating means and G. means for correlating said evaluating means to the scanning motion of said primary electron beam. 